1. Field of the Invention
The present invention relates to a method of correcting image signals read from an image in a case where an image is optically read by a linear sensor composed of CODs (Charge Coupled Devices) or the like and to an apparatus therefor.
2. Description of the Related Art
The method used for correcting dark current of a solid-state image pick-up device, until the present time, is described in Japanese Patent Laid-open No. 123617/1978. In this method, a light source is turned off to obtain the dark current and the value thereof is stored in a memory. Thereafter, the light source is turned on. The stored value of the dark current is subtracted from a signal at the time the image is actually read, and this is assumed to be a corrected image signal read from the image. In this method, however, there is a problem in turning the light source on and off. Another problem occurs when the value of the dark current is changed by variations in the surrounding environment such as temperature. This is due to the fact that once the value of the dark current is stored, it becomes the point of reference for future corrections regardless of the variations, thereby precluding accurate corrections.
In a correcting method described in Japanese Patent Laid-open No. 180377/1985, a dark current is represented by data of one dummy pixel. This dark current value is subtracted from an image signal made from an image and it is assumed to be a signal read from the image. In this method, however, since the dark current value is obtained by using one pixel, there are drawbacks in that a variation in each pixel and errors in measurement due to noise cannot be handled completely and therefore an accurate correction of the dark current cannot be performed.
As described above, since the variations in the photoelectric characteristics of the conventional solid-state image pick-up device are great at the dark time level, an accurate correction is not performed. In this regard, the linear sensor, in particular, has a drawback in that a streak appears running in the direction of the auxiliary scanning.
FIG. 1 shows an example of an apparatus (Japanese Patent Laid-open No. 333839/1988) which solves the above-mentioned problems. Signals read by a linear sensor 1 are converted to digital signals RS (e.g., 8 bits) and output by an A/D converter 3 through an amplifier 2. The digital signals RS are then input to an adder 4 and a subtracter 7. An addition signal AS (e.g., 11 bits) in the adder 4 is input to a latch circuit 5. Eight (8) bits of them are added in synchronization with a pixel clock PC shown in FIG. 2 from a timing generating circuit 8. This addition result LS (e.g., 11 bits) is input to a latch circuit 6 at a subsequent stage. Each of the latch circuits 5 and 6 is cleared by a shift pulse LSP. The latch circuit 6 sets the addition result LS of the latch circuit 5 in the latch circuit 6 in accordance with the addition result set pulse SP shown in FIG. 2 and inputs the average value of addition data, i.e., an 8-bit signal ALS, is input to the subtracter 7.
In this embodiment, the status of the pixels of the linear sensor 1 is as shown in FIG. 3. If eight pixels are read from the beginning of a line by the latch circuit 5, the output data of several pixels of blind pixels BC can be read and added, and an average value ALS of the 8 pixels of the blind pixels BC is output each time one line is scanned. Therefore, in a case where an image is read by the linear sensor 1 using the read pixels RP, the read signal is input to the subtracter 7 through the A/D converter 3. The average value ALS is subtracted from the read signal RS so that the dark current is corrected. This corrected signal is output as a signal PR read from the image.
In the above-described method, however, when errors in measurement occur due to noise, they are erroneously recognized as streaks running in the auxiliary scanning direction.